This invention relates to a black matrix type color picture tube capable of improving brightness and contrast without impairing beam landing tolerance.
A color picture tube generally comprises an envelope made up of a cylindrical neck, a funnel with its small diameter side connected to the neck, and a face plate sealed to the large diameter side of the funnel. An electron gun assembly of three electron guns is contained in the neck and a fluorescent screen is coated on the inner surface of the face plate. Confronting the rear side of the fluorescent screen is disposed a color selection electrode in the form of a shadow mask, for example. The electron beams emitted from respective electron guns transmit through apertures of the color selection electrode and then impinge upon the fluorescent screen to generate three colors.
The fluorescent screen of a color picture tube comprises stripes or dots in which phosphors that luminesce three primary colors, that is, green, blue and red, are arranged with light absorbing layers interposed therebetween. In the following, a stripe type fluorescent screen will be described but it should be understood that a similar description applies also to a dot tube.
FIG. 1 is a sectional view showing one example of the fluorescent screen of a prior art color picture tube. A plurality of stripe films 1b of such light absorbing material as graphite are formed on the inner surface of a face plate 1a with a suitable spacing between adjacent films 1b to act as the light absorbing layers. Three color phosphor stripes 1c (green), 1d (blue) and 1e (red) are formed on the stripe films 1b to respectively bridge adjacent stripe films and fill the space therebetween. An aluminum film 1f is applied on the phosphor stripes to act as a light reflecting metal back layer. The fluorescent screen having this construction is the so-called negative guard band type matrix fluorescent screen, and the gaps between the light absorbing layers 1b are termed matrix windows (in the case of the dot type fluorescent screen, matrix openings). As the gaps are increased, the amount of light emitted by the phosphors and derived out to the front side of the fluorescent screen increases, thereby increasing the brightness thereof. However, as the gap is increased, the difference between the diameter of the electron beam and the gap width decreases with the result that the tolerance of the beam landing decreases. Consequently, there are caused such problems of beam landing as clipping tolerance (the beam of one color collides upon the phosphor of other colors) and leaving tolerance (a beam partially leaves a given phosphor to be luminesced thereby). Moreover, since phosphors of respective colors are in the form of white powders having a percentage of reflection of about 85%, as the width of the window increases, the body color of the fluorescent screen approaches white, thereby impairing the appearance. On the contrary, when the width of the window decreases, the picture becomes dark, thus decreasing the commercial value.